It doesn't tell you how that data was collected. The entire gaming community didn't fill out forms to say who they are, it might be run off payment details or something in which case it can't account for kids getting stuff bought for them.

It came from the companies themselves. When you sign up for e.g. xbox gold you give them some information about yourself including gender. It's the same way they get information about age and pretty much everything else.

I've posted this once already, but it's relevant here too. www.theesa.com/facts/pdfs/ESA_EF_2012.pdf
As of 2012 adult women made up about 48% of gamers, though I this this took into account casual games if you take that away 31% is still a pretty conservative estimate.
The game companies who gave their data included Microsoft, Sony, Nintendo, EA, Ubisoft and a **** load of other companies.

The thing that makes interactive media so fantastic is their ability to appeal to anyone, to suck the player into a whole new world in a way that books or films fail to, so even new players have a level of emotional investment and involvement that keeps them hooked 'til the end.
Games aren't a gendered medium. No one says "films are for men" or "books are for men" so why would people say it about video games? Sure some genres are tailored to appeal to men over women, but that doesn't mean that women don't still enjoy those games or that there aren't other games out there better suited to women.
To be honest the biggest thing I think that turns women off a particular game isn't the genre, it's the protagonist. A lot of people play games as a form of escapism and when the protagonist is explicitly not what you are it makes the empowerment fantasy a little tricky, but there are more than enough women out there that can ignore that detail and there are more than enough games (increasingly, it seems) that allow for player choice on the matter.

Sorry for the rant, I get kind of excited when it comes to the place video games has in our culture and I do carry on a bit.